`NewDataBuilder` allocated `make([]byte, 0, req.FileSize)` using the
client-supplied `int64` with no upper-bound check. The DRPC 4 MiB wire
cap limits message size but not the integer value, so a crafted message
with `FileSize = 1<<40` forces a 1 TiB allocation, triggering an
unrecoverable `runtime.throw` that kills the entire `coderd` process.

Add a `MaxFileSize` constant (100 MiB, matching `HTTPFileMaxBytes` in
`coderd/files.go`) and reject negative or oversized `FileSize`, plus
negative or excessive `Chunks`, before the allocation.
`BytesToDataUpload` also returns an error for oversized data to preserve
the encode/decode round-trip contract. Fix a pre-existing reversed
subtraction in the `Add()` overflow error message.

Closes https://linear.app/codercom/issue/PLAT-231

<details>
<summary>Implementation details</summary>

- `provisionersdk/proto/dataupload.go`: New exported `MaxFileSize`
constant; validation in `NewDataBuilder` and `BytesToDataUpload`. Fixed
reversed subtraction in `Add()` error.
- `provisionersdk/proto/dataupload_test.go`: New
`TestNewDataBuilderValidation` with 7 subtests.
- Updated all 5 callers of `BytesToDataUpload` for new error return.
- Audited all `make([]byte, ...)` in provisioner paths; no other
client-supplied sizes.

</details>

> Generated by Coder Agents on behalf of @f0ssel

(cherry picked from commit a2e1ddb)